The Sodium-Hydrogen Exchanger Regulatory Factor-1 (NHERF-1) is an adaptor protein containing two protein-interactive PDZ domains and a C-terminal ERM binding domain that localizes to the brush border membrane of renal proximal convoluted tubule cells and binds to Npt2a, the major sodium-dependent phosphate transporter. Our recent studies have indicated that sodium-dependent phosphate transport in proximal tubule cells from NHERF-1-/- kidneys are resistant to the inhibitory effect of Parathyroid Hormone (PTH). In this current application, we explore the hypothesis that PTH mediates the phosphorylation of specific residues in PDZ I of NHERF-1 thereby regulating Npt2a/NHERF-1 complexes, the abundance of Npt2a in the apical membrane of renal proximal tubule cells, and as a consequence, the tubular reabsorption of phosphate. Elucidation of the factors that regulate the binding of target proteins to PDZ I of NHERF-1 may also provide broader insights into how regulation of PDZ domains of adaptor proteins impact on biologic responses to hormones and on the pathophysiology of NHERF-1 related diseases. In intact animals and cultured proximal tubule cells, we will use physiologic, biochemical, and cell biologic assays to determine how PTH-mediated phosphorylation of PDZ I of NHERF-1 regulateS the binding affinity of target proteins such as Npt2a and the proximal tubule reabsorption of phosphate. We propose three specific aims. First, we will map the serine and/or threonine residues in PDZ I of NHERF-1 that are phosphorylated in response to PTH and downstream protein kinases. Second, we propose to study the association and dissociation of Npt2a/NHERF-1 complexes in response to PTH-mediated phosphorylation of NHERF-1 using in-vitro and in-vivo assays. Third, we will determine the physiologic role of PTH-mediated NHERF-1 phosphorylation on the regulation of phosphate transport in the proximal tubule of the kidney. PUBLIC HEALTH RELEVANCE NHERF-1 is an adaptor protein that binds multiple transporters in the kidney including Npt2a, the major renal proximal tubule phosphate transporter. We will study the hypothesis that Parathyroid Hormone-mediated regulation of renal phosphate transport involves regulation of the binding of Npt2a to NHERF-1 by site-specific phosphorylation of the PDZ I domain of NHERF-1. These observations may provide mechanistic insights into the processes that regulate the binding of target proteins to adaptors such as NHERF-1 and provide broader insights into the pathophysiology of NHERF-1 related diseases.